The cultivated white button variety of Agaricus bisporus, known as A. bisporus (Lange) Imbach (syn. A. brunnescens Peck), is the predominant mushroom species in cultivation in the world today. After many years in which commercial mushroom sales in the United States were restricted primarily to white button A. bisporus mushrooms, there has been a recent trend toward increased sales of brown A. bisporus mushrooms of various types and other so-called exotic strains (species other then A. bisporus), as such mushrooms have increased flavor relative to the bland taste of white button mushrooms. However, many of these more flavorful and exotic tasting mushrooms are difficult to produce commercially or can only be collected in the wild. For example, Enoki mushrooms, also called enokitake (Flammulina velutipes), originated in Japan where they were gathered in the wild, although in the United States they are cultivated on live or dead tree trunks, tree roots, or branches that have been covered with soil. Shiitake mushrooms (Lentinus edodus), also known as Japanese black forest mushrooms, have been commercially cultivated and are widely available either fresh or dried in supermarkets as well as in Asian markets, although their commercial production is more difficult than that of A. bisporus, which grows in easily harvested beds. Originally harvested from hardwood trees in their native country for at least two thousand years, shiitakes are often cultivated on artificial logs made from sawdust. Morels (Morchella esculenta) are gathered in the wild in wooded areas in the spring. Scandinavians refer to morels as “truffles of the north.” Chanterelles (Cantharellus cibarius) grow in the wild in the Pacific Northwest in forests with pine trees and deciduous trees. Truffles (Tuber aestivum), perhaps the most famous fungus in the world and certainly the most expensive, are fungi that grow underground in wooded areas. They have never been successfully cultivated and are a challenge to forage in the wild. They can only be located by dogs or pigs that have been are specially trained to recognize the scent of the truffle. Black truffles from France, known as Perigord, are best known for flavoring pate de foie gras. White truffles from the Alba region of Italy are also available.
As should be apparent from the description above, commercial production of many exotic strains is difficult. This difficulty in commercial production has been overcome in part by developing strains of A. bisporus with similar colors, appearances, and flavors that can be used as a replacement for the exotic or wild mushroom strains described above. These “exotic” A. bisporus strains can be appreciated by discriminating consumers while still being capable of production in the standard commercial processes developed for A. bisporus production. Crimini (or cremini) is an Agaricus bisporus strain, similar to the familiar white A. bisporus mushroom strains found in most grocery stores, but it has a brownish color and is denser in texture with a pronounced earthy flavor. Portabellas (also spelled portobellos), which have only been widely available since the 1980s, are sometime thought of as an Italian strain of mushrooms but are actually large criminis that have been allowed to grow for longer periods of time. Because of their longer growth time, portabellas have a distinctly pungent, earthy flavor and fleshy texture and have seen increased use in recent years, often as a substitute for meat in vegetarian dishes, in addition to being used in side dishes or sauces for their own distinctive flavor.
Both white and brown strains of Agaricus bisporus—being varieties of the same species—have the same complex genetics and unusual biology. A. bisporus produces predominantly two spores per basidium, in contrast to most basidiomycetes fungi, which produce four spores per basidium. With four spores per basidium, each spore receives one of the four haploid nuclei produced by meiosis and germinates to form a haploid mycelium (a homokaryon). In A. bisporus each of the two spores typically receive two post meiotic nuclei referred to as “a” and “b”. There is good evidence (Evans H. J., in Chromosoma 10 115-135 (1959)); Summerbell, R. C., Castle, A. J., Horgen, P. A. & Anderson, J. B. in Genetics 123 293-300(1988)) that A. bisporus spores derived from two-spored basidia preferentially contain nuclei of complementary mating type. These spores germinate to produce diploid, self-fertile mycelium, known as heterokaryons, which contain the two nuclei a and b. This self fertile heterokaryon can, under the correct environmental conditions, undergo several fruiting cycles commonly referred to as “breaks.” A crop of mushrooms comprises the total yield from several successive breaks.
In addition to self-fertile spores, viable non-self fertile spores are produced at the rate of 1 to 20%. These homokaryotic spores arise from aberrant three- and four-spored basidia. The homokaryotic mycelium derived from these spores can be used for the controlled crossing that is the foundation of A. bisporus breeding. A traditional Agaricus breeding program utilizes the fact that homokaryons grow more slowly than heterokaryons. This permits the screening of large populations of spores for suitable parents, which can then be used in controlled crosses (Kerrigan, R. W., Baller, L. M., Horgen, P. A. & Anderson, J. B), in Mycologia 84 575-579(1992). This approach was used successfully by G. Fritsche (described in The Mushroom Journal 122 49-53 (1983) and in Genetics and Breeding of Agaricus, Chapter 1, 3-20, Pudoc (1991)) to develop the strains U1 and U3. Since their release in 1983, these strains have dominated the industry, either as U1, as U3, or as derivatives sold worldwide by numerous spawn companies (Castle, A. J., Horgen, P. A. & Anderson, J. B., in Applied and Environmental Microbiology 53 816-822 (1987); Loftus, M. G., Moore, D. & Elliott, T. J., in Theoretical and Applied Genetics 76 712-718 (1988)).
These U1 and U3 hybrid strans and their typical progeny are the white button mushrooms commonly found in grocery stores. Brown Agaricus bisporus mushroom strains of the portabella type that are currently available for commercial use include, but are not limited to, Sylvan SB65, Lambert 800, Lambert 801, Sylvan 295, Amycel 2400, and Amycel Bella. These are genetically related strains of a general class often referred to as “old fashioned browns” in the industry. Any improvement in color, appearance, flavor, and/or production values of these strains would be commercially advantageous.
Recent developments in mushroom genetics have allowed new strains of mushrooms to be developed via crossing, although there is still too much complexity in the genetic picture for crossing of untested strains to occur in a predictable manner. Most of the initial crosses have either the characteristics of the parents and thus do not represent improvements or have unexpected characteristics that are less desirable than those of the parent strains. However, once a desirable strain has been identified and developed, cell cultivation techniques allow commercial production of genetic clones via mushroom spawn, and the identification of genetic markers in the new strain allows its desirable characteristics to be followed into its progeny and used to select desirable strains from future crossings. At least one new mushroom strain has been the subject of a U.S. plant patent (No. Plant 7,636), while utility patents have issued on specific strains with improved characteristics (e.g., U.S. Pat. Nos. 4,996,390, 5,304,721, and 5,832,659).
All brown strains commonly available for sale in the U.S. are genetically identical, except the hybrid Sylvan 600 (syn. X618). There are a number of advantages for introducing greater genetic diversity into commercial brown mushroom production. The genetic homogeneity of the commercial brown crosses is especially problematic if a novel crop pathogen emerges which causes devastating crop losses. Since nearly all of the commercial brown strains are identical, they would be equally affected.
Additionally, cap color is one of them most important economic physical characteristic of A. bisporus strains. Currently available brown strains of A. bisporus all have generally the same (more or less) brown tone, and the development of brown strains that have a darker color would be economically advantageous.
Accordingly, there is a need for new mushroom strains having genetic diversity as well as improved appearance, flavor, and/or production characteristics that that exceed those of existing portabella mushrooms. By the introduction of wild mushroom germ plasma into commercial mushroom strains, we have a developed a novel breeding pedigree. The products of our pedigree give brown mushrooms of the portabella and crimini varieties having good flavor and appearance as well as having excellent production values.